1. Field of Invention
This invention relates to extracting a selected frequency component in rectangular coordinates from each waveform of a set of electrical waveforms. More particularly, it is directed to a method and apparatus for obtaining the rectangular coordinates of a selected frequency component such as the 60 Hz component from voltage and current waveforms of a polyphase transmission line using the full cycle Fourier algorithm.
2. Background Information
In some applications it may be necessary to obtain polyphase voltage and current information in terms of rectangular coordinates or vectors. On such application is in certain transmission line relaying equipment where it is desirable to obtain 60 Hz three-phase voltage and current information in rectangular coordinates. One way of obtaining this information is to simultaneously sample each of the waveforms at equally spaced times in one power cycle. This technique is known as the "full cycle Fourier algorithm".
According to current practice of implementing this algorithm, each input signal representing the instantaneous analog value of one parameter (a voltage, or a current converted to a proportional voltage) is applied to a separate sample/hold circuit. The sample/hold circuit is typically an integrated circuit consisting of an input buffer, an output buffer and a switch. At the sample time, a microprocessor switches the sample/hold control line from sample to hold. The sample/hold circuits then contain the voltage applied to their inputs at the time the control line is switched. Next, the microprocessor sequentially selects each sample/hold output through the control lines of a multiplexer which sequentially applies the inputs to an analog to digital converter to digitize each selection for input to the microprocessors. Thus, while the samples are serially input to the microprocessor, they represent the instantaneous values of each of the sensed parameters at the same instant of time.
The microprocessor utilizes these samples in the execution of the full cycle Fourier algorithm. An essential feature to this technique is that all of the waveforms are sampled simultaneously. This requires a separate sample and hold circuit for each parameter.
It is the primary object of the present invention to reduce the hardware required to extract a selected frequency component in rectangular coordinates from a set of electrical waveforms.
It is a further object of the present invention to achieve this primary object through a novel method and apparatus for implementing the full cycle Fourier algorithm.